Various applications require the deposition of thin, smooth and uniform coatings on substrates. Where the application necessitates deposition on all sides of the substrate, it has been found desirable to place the substrate in a thermionic emission of the coating material induced by magnetron sputtering or plasma spraying. Substrates such as microballoons have been coated using low pressure plasma sources by bouncing the substrates or by gas jet levitation. However, if the targets are bounced during the deposition process, the coating can be damaged or the targets tend to adhere to the bouncing plate or to each other due to either electrostatic interactions or cold welding. Levitation processes using gas jets during the coating process are hampered due to the electrostatic interaction between the target and the levitator, while the low pressure required for sputtering hampers the levitation process.
Plasma coating processes heretofore have also been carried out at similar low pressures principally because it is under these conditions that a plasma is not influenced by local thermal equilibrium (hereinafter referred to as "LTE") conditions. A plasma exhibiting non-LTE contains many heavy species (e.g. atomic free radical and excited molecular, in ground, ionized or excited states) of the coating material at temperatures lower than the electron temperatures, significantly facilitating the carriage of the coating material in the plasma to the substrate and rendering unnecessary periodic quenching of the plasma. Unfortunately, at the low pressures conducive to non-LTE conditions, it is often difficult to levitate substrates, and the rate of deposition and yield are unacceptably low.
Increasing the plasma operating pressure to the relatively high pressure of at least approximately 0.1 atmosphere may result in a more extensive LTE influence, depending upon arc current utilized, as well as species in the plasma (low ionization or molecular species). Although levitation of substrates is easier, and deposition rate, yield and heat conductivity are all greater under these conditions, few, if any, atomic and free radical species of the coating material exist in the plasma, deleteriously affecting the carriage of the coating material to the substrate. Accordingly, existing plasma deposition apparatus have been designed to operate at pressures of only thousandths of an atmosphere to foster non-LTE conditions. Additionally, high pressure plasma deposition apparatus almost universally has been in the form of a "spray gun" often with a complex combination of multiple chambers.
In recent years a type of arc plasma source radically different in structure than that previously employed as sources for plasma depositions has been developed for use as light source in spectroscopy. This source provides a horizontal or vertical rotating arc burning in a graphite tube in air immersed within a magnetic field to increase the residence time of particles in the plasma, for better analysis. We have found that a modified form of this source may be utilized to establish and maintain a non-LTE plasma at the relatively high pressures noted above and facilitate the deposition of substrates with coatings.